Medical diagnosis that utilizes ultrasound imaging has recently been widely used in medical fields. The ultrasonic image is obtained by emitting ultrasonic wave from an ultrasonic probe toward a necessary body part and detecting ultrasonic echo from the body part as electric echo signals by use of an ultrasound observing device that is connected to the ultrasonic probe through a connector. It is also possible to obtain an ultrasonotomographic image by scanning with the ultrasonic wave.
As a concrete example of medical diagnosis utilizing ultrasound imaging, staging of esophagus cancer or stomach cancer based on ultrasonotomographic images has been disclosed, for example, from “Staging Of Esophagus Cancer With Fine Probe Ultrasonic Endoscope” by Hiroya OZAWA in DIGESTIVE ENDOSCOPE Vol. 14, No. 5, 2002. In this type of diagnosis, medical treatment for the patient is decided depending upon invasion degree of cancer especially into the sub mucous tissues, which are divided into three submucous tissue (sm) layers, as shown in FIG. 12.
However, because the mucosal (m) layer as laying on the muscularis mucosae (mm) and all the submucous tissue layers give rise to very strong high-echo that can often cause chroma clip in highlight range, that is, detail is lost or bleached out in high luminance range of subsequent ultrasonotomographic image. As a result, the muscularis mucosae and the three layered structure of the submucous tissues, which is referred to for checking if the cancer metastasizes to the lymph nodes or not, become indefinite on the image, so that it is hardly possible to make adequate diagnosis.
Meanwhile, an X-ray inspector has been suggested in U.S. Pat. No. 5,708,693 wherein an input image is decomposed into several images of different frequency bands. The images of different frequency bands are filtered through sequential statistic value filtering, and the filtered images are composed to obtain an output image. This is for the sake of reducing noises from the X-ray radiograph. Also an ultrasonic imaging device has been suggested in United States Patent Application Publication no. US2004/0073112, wherein structure-enhanced image data and texture-enhanced image data are extracted from an ultrasonotomographic image, and these image data are synthesized after being weighted respectively. In the structure-enhanced image data, the structure of living body tissues is enhanced. In the texture enhanced image data, texture pattern resulting from properties of living body tissues is enhanced.
Indeed the prior art disclosed in JP-A 10-505443 can reduce the noises from the X-ray radiograms, but data on details of the images, such as contours, remain as they are. The prior art disclosed in United States Patent Application Publication no. US2004/0073112 is designed to improve image quality by balancing in the image between the structure of the living body tissues and the texture resulting from the properties of the living body tissues. Therefore, both of these prior arts cannot hold down the bleaching in images of the mucosal layer and the submucous tissue layer.
Ordinary image processing, such as gain control and contrast control, may be useful for holding down the occurrence of the chroma clip in highlight range to some extent. But with such ordinary image processing, images of deeper layers of the living body, which are originally darker than other portions, would be more darkened together with other portions, causing chroma clip in shadow range. As a result, details are lost or blacked out in low luminance range of the subsequent image, so that the ultrasonic image as the whole becomes difficult to diagnose.